Mössbauer comparison of Fe(100)/Ni and Fe(110)/Ni heterostructures grown by molecular beam epitaxy (abstract)

Abstract

Epitaxial Fe(100)/Ni and Fe(110)/Ni heterostructures were grown using a Perkin-Elmer PHI 430B molecular-beam-epitaxy system equipped with (RHEED) and quadrupole mass analysis. The growth system typically achieved a base pressure of less than 5×10−10 Torr, and a growth pressure of less than 3×10−9 Torr. Typical growth rates were 3 Å/min for Fe and 2 Å/min for Ni. For all the heterostructures, the Ni thickness was held at 14 Å, the number of repetitions varied between 8 and 15 cycles, and growth always began with the Fe bilayer. Protective Ag covers were grown on all films. Three Fe (100)/Ni heterostructures were grown on 5-kÅ single-crystal Ag(100) bases grown on NaCl(001).1 The single-crystal Fe(100) bilayer thicknesses were 3, 8, or 12 monolayers (ML). The substrate growth temperature for this series was ramped from 40 to 80 °C due to radiant heating from the effusion cells. Four Fe(110)/Ni heterostructures were grown with Fe bilayer thicknesses of 2, 4, 8, and 12 ML. These heterostructures were grown on 5-kÅ Ag(111) single-crystal bases grown on single-crystal natural muscovite mica. An intervening epilayer of NaCl (150 Å) deposited between the mica and Ag base facilitated film removal from the Fe-contaminated mica for ex situ transmission 57Fe Mössbauer analysis. The substrate growth temperature for this series was held at 180 °C, since this appears to be optimal for Fe(110) growth on Ag(111).2 Note that the resultant Fe(110) growth is mosaic with Fe[001] parallel to Ag〈110〉 (threefold symmetry). The RHEED observation of the growth of Ni on Fe(100) always resulted in the Ni RHEED pattern closely following that of the Fe (100) pattern, with broader Ni RHEED lines apparent. The characteristic behavior of our Ni RHEED patterns mimicked that observed by Heinrich et al. for bcc Ni(100),3 and did not match that of fcc Ni. The Ni-on-Fe(110) growth was analogous in RHEED characteristics to that of the (100) case. The Ni RHEED patterns again closely matched that of Fe(110), the only real difference being the broadening of the Ni RHEED streaks. Note that fcc Ni(111) was seen to grow on Ag(111) under similar growth parameters. It is likely that a metastable bcc Ni(110) structure analogous to bcc Ni(100) was observed. The quality of the Fe/Ni RHEED patterns did not seem to significantly worsen from bilayer to bilayer throughout the growths of either series. Furthermore, the respective Ag cover layers for all films showed excellent RHEED patterns. All the observed Mössbauer spectra for both series of Fe/Ni multilayers show sextets at room temperature, except for the 2-ML Fe(110) film, which exhibited a very small additional single-line central feature. At 4.2 K, the 2-ML Fe(110) film had no change in central feature, ruling out superparamagnetism as a cause. All films exhibited in-plane magnetization, and thinner Fe bilayers exhibited a growing isomer-shifted second sextet-site presence, suggestive of an interfacial Fe site at the Fe/Ni interface. An enhanced hyperfine field is seen for the thinnest Fe bilayer films at 4.2 K. This enhancement is greatest for the Fe(100) system [most enhanced Fe(100) site=365 kOe vs most enhanced Fe(110) site=351 kOe, compared to 341 kOe for bulk]. The thickest Fe bilayer films for both series showed nearly-single-site, bulklike hyperfine-field behavior. The Mössbauer spectra observed for these epitaxial Fe/Ni heterostructures are different than that previously reported for polycrystalline fcc Fe/fcc Ni films.4 More detailed structural and magnetic studies of the novel bcc Ni reported here should be pursued.